Fields of Embodiments
The disclosure relates generally to the field of producing hydrocarbons from a formation.
Description of Related Art
This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Substantial volumes of hydrocarbons exist in low-permeability and high-permeability formations around the world. Low-permeability formations may be formations that are near horizontal wells with multiple fracture stimulations distributed along the well and required to produce fluids from the formation at economic rates. For example, low-permeability formations may be less than or equal to 10 millidarcies (mD) while high-permeability formations may be formations that are greater than 10 mD. Low-permeability formations may be predominantly sandstone, carbonate, or shale and/or may have some high-permeability streaks. High-permeability formations may have some low-permeability streaks. From a practical perspective low permeability reservoirs may require horizontal wells with one or more hydraulic fracture stimulations to achieve economic production rates while high permeability reservoirs may be economically exploited with vertical or horizontal wells and may not require hydraulic fracture stimulations.
During primary production natural reservoir energy drives hydrocarbons from the reservoir and into the wellbore. Initially, the reservoir pressure is considerably higher than the bottomhole pressure inside the wellbore. This high natural differential pressure drives hydrocarbons toward the well. During primary production the reservoir pressure declines as fluids are removed from the formation. The natural reservoir energy exploited in primary production such as oil and water expansion, evolution and expansion of gas initially dissolved in the oil, and rock compaction have limited ability to compensate for the volume of produced hydrocarbons and thereby to mitigate the pressure decline. As the reservoir pressure declines because of production, so does the differential pressure between the reservoir and wellbore, resulting in declining production rates. Primary production ends when the pressure is so low that the hydrocarbon production rate is no longer economical. Recovery during primary production is typically less than 15%. The lower the permeability of the formation the more difficult it is for pressure and fluid to be transmitted towards the well. This results in lower initial rates, more rapid pressure decline, and lower recovery of hydrocarbons.
Production of hydrocarbons from high-permeability formations often results in more satisfactory recovery rates than low-permeability formations. The recovery rate of hydrocarbons in high-permeability formations can be as high as 75%. To achieve these higher rates, different drive mechanisms may be used. For example, water injection or gas injection may be used to provide pressure support and to displace hydrocarbons. Other processes, such as injecting miscible gases, surfactants, solvents, polymers, or steam may also be used to help improve hydrocarbon recovery.
To increase the recovery rate of hydrocarbons during primary production from low-permeability formations, operators have tried using various well types and configurations, different well stimulation methods and processes that exploit different drive mechanisms during and after primary production. For example, operators have tried closely spaced vertical and horizontal wells, wells that have been stimulated using a variety of methods such as hydraulic fracturing, acid injection or acid fracturing. Stimulation methods increase the productivity of a well, enabling a well to initially produce hydrocarbons at a higher rate. Additionally, operators have tried some of the same drive-mechanisms used in high-permeability formations, such as water-flooding or gas-flooding, after fracturing during primary production. One well design that is commonly employed in low permeability formations, as shown in FIG. 1, consists of installing a horizontal well 1 and creating fractures 2 that emanate from the wellbore 5 of the well 1 to recover the hydrocarbons. As shown in FIG. 2, stimulated horizontal wells can be utilized for water-flooding by a method that entails operators installing a well 100 and injecting water so that the water displaces hydrocarbons toward producer wells 4, 204. Gas-flooding is similar to water-flooding, but entails injecting gas into a well instead of water to displace hydrocarbons to a production well.
Although fracturing can help primary production from a low permeability formation to be more economically attractive by increasing initial production rates, the process has two major disadvantages. First, due to rapid pressure decline in the wellbore region, the production rate of recovered hydrocarbons typically declines quickly to less than 25% of the initial rate of recovery within a year. Second, the total percentage of recovered hydrocarbons relative to the hydrocarbons contained in the formation is low. Often, the total percentage of recovered hydrocarbons is less than 15%. The low formation permeability and resulting low rate of pressure diffusion through the reservoir, results in rapid pressure decline at the well and rapidly declining production rates of hydrocarbons. Furthermore, since primary production processes rely on fluid expansion as their drive mechanisms they tend to have very low recovery levels in all oil reservoirs.
Disadvantages also result when operators use water-flooding or gas-flooding after using fracturing during primary production in a low-permeability formation. These processes have the potential to increase recovery of hydrocarbons to 20% or more. However, they require the drilling and fracturing of additional injection wells or the conversion of existing production wells into injection wells. Because of the low permeability, the injection wells need to be relatively close to the producing well to provide sufficient pressure support and achieve economic rates. Nonetheless, water-flooding in low-permeability formations is often limited by low injection rates due to the low-permeability formation, injection pressure constraints, plugging, separation between the wells and relative permeability effects. A key limiting factor is that if the injection wells are placed in close proximity to the production wells, the fractures from the wells may intersect. This results in high conductivity pathways between the wells that severely limit the rate of hydrocarbon production and the overall recovery that can be economically achieved. Gas-flooding in low-permeability formations is often limited by poor sweep due to gravity override, viscous fingering and heterogeneity contrast. These detrimental effects often cause fractures to intersect, thereby eliminating the pressure difference needed for sweep to occur. These disadvantages are often exacerbated in low-permeability formations because of tight well spacing and higher permeability streaks.
Additional disadvantages may also result when the aforementioned drive mechanisms are used in low-permeability or high-permeability formations. The effectiveness of water injection for improved recovery is sometimes adversely affected by reduced injectivity due to plugging of injection wells with solids, scale, oil, etc. Enhanced recovery techniques, such as injection of miscible gases, surfactants, solvents, polymers, modified brines, or steam can sometimes be applied to high permeability reservoirs to improve recovery, but the use of these techniques is often uneconomic. There is a significant time difference between when these relatively expensive fluids are injected into an injection well when that incremental hydrocarbon production occurs at a producing well.
A need exists for improved technology, including technology that may address one or more of the above described disadvantages of conventional ways of producing hydrocarbons from a formation.